1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication method thereof, and more particularly to a semiconductor device comprising at least one conductive structure which has a refractory metal salicide layer, and a fabrication method thereof.
2. Description of the Related Art
In semiconductor technology, a metal-oxide-semiconductor (MOS) transistor is composed of three electrodes: a gate, a source and a drain. The early MOS transistor is composed of a metal layer, a silicon oxide layer and a silicon substrate. However, most of metals have bad adhesion to silicon oxide. Polysilicon with good adhesion to silicon oxide is provided to replace the metal layer. Polysilicon, however, has high resistance. Even if being doped, resistance of doped polysilicon is still too high, and the doped polysilicon cannot replace the metal layer of the MOS transistor in this aspect. A solution is provided later. A metal salicide layer with a thickness similar to that of the polysilicon layer is disposed on the polysilicon layer. The low-resistance metal salicide layer and the polysilicon layer constitute a conductive layer.
Metal salicide has high melting point, thermal stability and low resistance so driving current and operational speed of the device are improved. Therefore, metal salicide technology gradually has been applied in integrated circuit processes. Additionally, due to the shrinkage of the integrated circuit technology, a gate width of a device is also reduced. If the metal salicide is titanium salicide, the narrow-line-width effect may occur. It means that if the line width is reduced, the sheet resistance of the gate dramatically increases. Therefore, other materials, such as cobalt salicide (CoSi2) or nickel salicide (NiSi2), have been used to replace titanium salicide.
Since nickel salicide has low resistance, low process temperature and minor narrow-line-width effect, it has been widely used in the 65-nm MOS field effect transistor (MOSFET) technology. The thermal stability of nickel salicide, however, is poor. For the time being, nickel/platinum alloy salicide with high thermal stability has replaced nickel salicide.
Due to its high chemical stability, platinum is hard to be removed. Though a selected etch solution can remove nickel/platinum alloy, the etch solution may damage nickel/platinum alloy salicide. Accordingly, how to remove nickel/platinum alloy without damaging other parts of the device becomes an issue in this field.